Child/Parent event raising

Question

I have a parent abstract class with several children classes. Eventually, I would like the progress done in the children classes to be shown via a progress bar in the GUI.

What I currently have done right now, which I am realizing will not work, is the event method definition declared in the parent class as a virtual method which each child class will overwrite. So something like :

public abstract class Parent
{
 public event EventHandler someEvent;

 protected virtual void OnSomeEvent(object sender, EventArgs e)
    {
          EventHandler eh= someEvent;
        if (eh!= null)
        {
            eh(this, e);
        }
    }
}

And my child classes have something like :

  protected override void OnSomeEvent(object sender, EventArgs e)
    {
        base.OnSomeEvent(sender, e);
    }

and the event is raised somewhere in the child class.

However, seeing as the parent class is abstract, I will not be able to listen to the event from my GUI because I can not create an instance of an abstract class.

Am I completely off course and/or is there another method of doing this?

Solution

You can attach to the event from the child instance.

public abstract class Parent
{
      public event Action Something;

      public void OnSomething()
      {
          if (Something != null)
          {
              Something();
          }
      }
}
public class Child : Parent
{

}

Child c = new Child();
c.Something += () => Console.WriteLine("Got event from child");
c.OnSomething();
> Got event from child

You can even declare it as a Parent type that contains a child:

Parent c2 = new Child();
c2.Something += () => Console.WriteLine("Got event from Parent type");
c2.OnSomething();
> Got event from Parent type

An abstract class is just a code template that gets copied into every class that inherits from it (to put it simply). Think of it like, all of your Child classes contain an identical copy of the code that exists in Parent.

---

Note that this will also produce a unique event handler for each instance of Child. Having a static event handler for all Childs that derive from Parent would look like this, and requires no code in Child:

public abstract class Parent
{
    public static event Action Something;

    public static void OnSomething()
    {
        if (Something != null)
        {
            Something();
        }
    }
}

Then, you could do something like this, for example:

Parent.Something += () => Console.WriteLine("This will be invoked twice.");

Child c = new Child();
Child c2 = new Child();
c.OnSomething();
c2.OnSomething();

> This will be invoked twice.
> This will be invoked twice.

Both of those objects/event calls will invoke the same event handler even though they come from separate children.

Comments

[qJake]

Why can't your UI attach to the event on the child instance? If you inherit from Parent, Child will have the event, too.

[mo alaz]

The idea is that I don't necessarily want the GUI to know which child raised the event. @SpikeX

[qJake]

So you want a static event handler on Parent, that only has one instance, is what you're saying.

[mo alaz]

I want a progress bar for each child so your solution was just right @SpikeX

[Osama AbuSitta]

Possible duplicate of c# : accessing parent class's events not possible?

[mo alaz]

Just to be clear, what is your implementation of OnSomething() in the child class?

[qJake]

@moalaz There isn't one. It inherits from Parent automatically. You can invoke OnSomething(); from within Child, unless you specifically want to override it with another behavior, but by default, it will just invoke the Something event.

[qJake]

@moalaz I also added some code for a static event if you didn't want the event handler to be unique to each Child instance.